When a network develops toward a broadband and mobile system, a 3rd Generation Partnership Project (3rd Generation Partnership Project 3GPP) proposes a Long Term Evolution (Long Term Evolution, LTE) solution, namely, an Evolved Universal Mobile Telecommunication System Terrestrial Radio Access Network (Evolved Universal Mobile Telecommunication System Terrestrial Radio Access Network, E-UTRAN), for a mobile access network, and a System Architecture Evolution (System Architecture Evolution, SAE) solution, namely, an Evolved Packet Core (Evolved Packet Core, EPC), for a mobile core network. In LTE/SAE architecture, because some operators expect that, after an Evolved Packet System (Evolved Packet System, EPS) network is deployed, a voice service is still provided by a conventional 2G/3G Radio Access Network (RAN) and a Circuit Switched (Circuit Switched CS) domain core network while the EPS network provides only Packet Switched (Packet Switched, PS) domain data access, the EPS network coexists with the conventional 2G/3G CS domain core network for a period of time. A User Equipment (User Equipment, UE) can access a CS domain core network through a 2G/3GRAN such as a Global System for Mobile Communications (Global System for Mobile Communications, GSM)/Enhanced Data Rate for GSM Evolution (Enhanced Data Rate for GSM Evolution, EDGE) Radio Access Network (Radio Access Network, GERAN) or a Universal Mobile Telecommunication System Terrestrial Radio Access Network (Universal Mobile Telecommunication System Terrestrial Radio Access Network, UTRAN), and access the EPC through the E-UTRAN.
To balance loads between RANs, namely, Radio Access Technology (Radio Access Technology, RAT) systems, an inter-RAT handover is supported. When a UE performs a CS domain handover between 2G/3G RAT systems, a source access controller to which a source cell belongs in a source RAT system can interact with an access controller to which a target cell belongs in a target RAT system through a RELOCATION REQUIRED message, and a RELOCATION COMMAND message or a RELOCATION PREPARATION FAILURE message.
In the above technical solutions, when an inter-RAT PS domain handover is performed or before an inter-RAT CS or PS domain handover is performed, the source RAT system is unable to acquire the load information of the target RAT system, so that inter-RAT load balancing fails to be implemented. As a result, communications quality of the systems fails to be guaranteed.